The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Switching power converters typically convert a DC voltage into an AC voltage by operating switching elements and then employ a rectifier and smoothing circuit to convert the AC voltage back to a DC voltage. A control circuit may control a duty cycle of the switching elements. Switching power converters allow for a variable output voltage by varying the duty cycle of the switching elements. The ratio of the output voltage and the input voltage is typically determined by the duty cycle of the switching elements.
As newer integrated circuits may provide for more electronic functions in smaller packages, it is essential for power converters to become smaller. One way to reduce the size of a power converter is to increase the operating frequency of the switching elements. However, as the frequency of the switching elements increase, the power loss of the power converter also increases. Increasing the frequency of the switching elements tends to increase electromagnetic interference generated by the power converter.
In order to minimize the disadvantages associated with increasing the frequency of the switching elements, it is desirable for the switching elements to turn on and off when the voltage and/or current of the switching elements becomes zero. By using power transistors as the switching elements, a parasitic output capacitance associated with the power transistors may be used to create a resonating LC circuit with a parasitic leakage inductance of an isolation transformer. The resonating LC circuit renders the current and voltage associated with the power transistors into a sinusoidal waveform forcing the current and voltage to pass through a zero crossing. The power transistors are then turned on and off at the zero crossing of the sinusoidal waveform. This minimizes overlap of falling current and rising voltage when the elements turn off and rising current and falling voltage when the elements turn on. This type of switching is often referred to as soft switching, whereas conventional switching is often referred to as hard switching.
In a conventional full bridge converter utilizing either type of switching, freewheeling current is generated in a rectifier portion of the converter when the switching elements turn on and off. The freewheeling current causes a relatively high reverse recovery current through one of the rectifiers when one of the switching elements turns on and through the other rectifier when the other switching elements turn on. The reverse recovery current causes voltage spikes at the blocking diode. The reverse recovery current also causes the temperature of the rectifier to increase, which correspondingly increases the reverse recovery current and corresponding voltage spike at that diode. Eventually, the voltage spikes rise to a level that exceed the rating of the typical output rectifier utilized on the secondary side of the transformer.
One attempt to limit the freewheeling current and corresponding voltage spikes have utilized a diode between the voltage rails at the output of the converter. The diode is placed in a location that is advantageous, as the voltage stress on the diode is substantially less than the voltage stress upon the output rectifiers, so that the diode may be used as a freewheeling diode. The output current flows through the freewheeling diode and consequently decreases or limits the reverse recovery current through the rectifiers when the switching elements of the bridge are all off. However, conventional output inductors or chokes do not insure that all output current flows through the freewheeling diode placed between the voltage rails. In a typical case, most of the freewheeling current flows through the output rectifiers rather than through the freewheeling diode. This leads to a higher reverse recovery current through the output rectifiers.
Another attempt to limit freewheeling current uses a secondary inductor having a first end connected to a primary inductor and a diode connected to a second end of the secondary inductor. The secondary inductor inhibits current flow through the pair of rectifiers when no voltage is applied to the primary side of the isolation circuit.
An alternative method to minimize losses due to reverse recovery current is by implementing a soft switch that switches at zero voltage on the leading leg switching elements and zero current on the lagging leg switching elements. Such circuits are often typically referred to as zero voltage zero current switching (ZVZCS) converters. A conventional ZVZCS full bridge converter is depicted in FIG. 1. The conventional ZVZCS converter uses a coupled choke N4 that is in phase with an output choke N3. A cathode of diode Dd is connected to a first side of N3. An anode of Dd is connected a first side of N4. A capacitor Ch is connected to the anode of Dd and a negative rail of the output circuit. A cathode of diode Dc is connected to a second side of N4, and an anode of Dc is connected to the negative rail of the output circuit. A cathode of Df is connected to the cathode of Dd and an anode of Df is connected to the negative rail of the output circuit. Ch is charged through a resonance leakage inductance of N4 during the active period. During the freewheeling period, Ch discharges causing current flowing through the primary side to reset to zero. Although this circuit is efficient, it requires many components, which limits the ability to minimize the size of the circuit.